Immobilization of bioactive peptides onto surfaces has been proven to be an effective avenue to improve cell attachment, influence proliferation, and direct differentiation in tissue engineering. Physical adsorption/encapsulation and chemical conjugation have both been applied to derivatize tissue engineering scaffolds with bioactive peptides. Most of these methods were developed for polymeric materials, while the surface decoration of inorganic surfaces has received less attention, due to the lack of diversity in presenting functional groups for highly efficient chemical reactions. However, many inorganic materials are useful in the medical applications field. For instance, titanium and zirconia are widely used in prosthetic devices and dental implants; cerium oxide nanoparticles are potent antioxidants in therapeutics; and iron oxide magnetic nanoparticles are used to enhance the magnetic resonance imaging contrast in disease diagnostics. Thus the development of efficient and convenient methods to immobilize bioactive peptides onto the surface of metal oxide materials (TiO2, ZrO2, CeO2, Fe3O4, etc.) will not only influence the cell behavior locally, but will also contribute to the improvement of diagnostic and therapeutic techniques in the clinic.
Titanium is the most widely used material in bone implants and dental fixations due to its low density, high strength and high resistance to erosion. In physiological conditions, the oxide passivation layer of 2-20 nm TiO2 is quickly formed on titanium implants. Several methods have been developed to decorate titanium implants with bioactive peptides/proteins. Modifications can be achieved through physical interactions, such as protein-encapsulated coating, erosion and subject protein adsorption, and peptide-grafted polycation adsorption. However, the diffusion of loaded bioactive components may require high doses, and lead to low drug efficiency, and other adverse reactions. Chemical conjugation by generating reactive functional groups using electrochemical anodization, acid-etching, and oxidation, have been utilized to covalently conjugate the bioactive moieties onto the titanium implant surface, but the methods require complicated procedures and change the surface properties of the device during fabrication.
The presence of 3,4-dihydroxyphenylalanine (DOPA), which is found abundantly in mussel adhesive proteins, has been connected to the strong adhesion of mussels onto multiple surfaces in wet conditions. Catechol group is the functional group of DOPA, which is known to interact with titanium oxide surface through coordination bond or H-bond with pH sensitivity. Catechol is also crosslinked together under oxidative or basic conditions to form coating layers on surfaces. Thus it has been served as adhesive building blocks in the surface coating of variety of materials, including metal oxides, and organic polymers. Besides titanium oxide, the interaction of catechol with other metal oxides has also been studied, including iron oxide, chromium(III) oxide manganese dioxide, aluminium oxide and zirconia. Anti-fouling ethylene glycol dendrons and glycocalyx layers have been successfully coated onto titanium oxide surfaces with catechol-functionalized oligomers as the surface-anchoring domain. However, sequestering bioactive moieties, such as peptides that are known to direct cell behaviors, using catechol-bearing molecules on the surfaces of biomaterials has not been reported.
Modular peptides are conjugated molecules containing several different peptide sequences that are known to have specific bio-functionality. In the modular peptides, there are two active components, the surface-binding peptide that sequesters the whole molecule on the surface and the bioactive subunit that influences the cell behavior. The loading concentration and retention time of the peptides on the surface are critical parameters that determine whether molecular signaling in the cell will be triggered. In many studies it was shown that the cell response to specific peptides is concentration-dependent. However, in most applications, the concentrations that are required to trigger and sustain the cell response are less understood. Strong adsorption is the prerequisite to realize efficient immobilization with bioconjugate solutions at low concentration, and to retain the peptides on the surfaces over extended periods.
It is known that if there are more than one pair of ligand-receptor interactions binding simultaneously, a synergistic augment rises in binding affinity with an order of magnitude enhancement over the corresponding monovalent ligand. This multivalent binding strategy has been used extensively in nature and with synthetic molecules to enhance their binding affinity. Dendrimers are ideal platforms to construct multivalent binding ligands due to their abundant functional groups in the periphery region. Studies have shown that the molecular structure of the multivalent ligands, including binding valency, the flexible linkage units, molecular architecture and receptor density all play significant roles in the ultimate association constant of the multivalent ligand with its receptor.
Osteogenic growth peptide (OGP) is an endogenous regulatory tetradecapeptide presents in mammalian serum with concentrations at the micromolar scale. Native or synthetic OGP regulates proliferation, alkaline phosphatase activity and matrix mineralization in studies of osteoblastic cell lines derived from human and other mammalian species. As its active portion, the carboxy-terminal pentapeptide, OGP(10-14) directs rat bone marrow mesenchymal stem cells to differentiate to osteoblasts. OGP or OGP(10-14)-functionalized biomaterials, including scaffolds for bone tissue engineering, gradient substrates, and peptide nanofibers, have been prepared, and shown to promote both cell proliferation and osteogenic differentiation, in vitro and in vivo.
What is needed in the art is a versatile molecule that will tether bioactive molecules to a variety of surfaces in such a way that their inherent biological function is preserved.